1. Field of the Invention
The present invention relates to an acoustic wave device used as, for example, a resonator or a band-pass filter, and more specifically, to an acoustic wave device that includes an insulator layer for improving temperature characteristics that is arranged so as to cover an interdigital (IDT) electrode and a method for fabricating the same.
2. Description of the Related Art
An acoustic wave resonator and an acoustic wave filter are widely used in a band-pass filter in a mobile communications device. One example of an acoustic wave device of this kind is disclosed in Japanese Unexamined Patent Application Publication No. 2004-112748. FIG. 13 is a front sectional view that schematically illustrates a surface acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2004-112748. In a surface acoustic wave device 101 illustrated in FIG. 13, an IDT electrode 103 is provided on a piezoelectric substrate 102. An insulator layer 104 is provided so as to cover the IDT electrode 103. Here, the piezoelectric substrate 102 is made of a piezoelectric material that has a negative temperature coefficient of frequency, such as a lithium tantalate (LiTaO3) substrate or a lithium niobate (LiNbO3) substrate. On the other hand, the insulator layer 104 is made of an insulating material that has a positive temperature coefficient of frequency, such as silicon dioxide (SiO2). Consequently, the surface acoustic wave device 101 having favorable temperature characteristics is provided.
In the surface acoustic wave device 101, the insulator layer 104 made of, for example, a SiO2 film, is formed after the IDT electrode 103 is formed. Therefore, when the insulator layer 104 is formed by a thin-film forming method, such as vapor deposition, a projection 104a and a depression 104b, are inevitably formed. This is because the IDT electrode 103 is present under the insulator layer 104 and thus the surface of the insulator layer 104 is raised at a region where the IDT electrode 103 is present, which results in the projection 104a. 
However, when the projection and depression are formed, an unwanted ripple appears in the frequency characteristics. An acoustic wave device that does not include any projections and depressions is disclosed in WO 2005/034347 A1. The structure of the acoustic wave device described in WO 2005/034347 A1 is illustrated in FIG. 14 with a schematic front sectional view. In an acoustic wave device 111, an IDT electrode 113 is provided on a piezoelectric substrate 112. A first insulation layer 114 having the same film thickness as the IDT electrode 113 is formed around a region where the IDT electrode 113 is present. A second insulator layer 115 is formed so as to cover the IDT electrode 113 and the first insulator layer 114. Here, after the first insulator layer 114 and the IDT electrode 113 is formed, the second insulator layer 115 is formed. As a result, the second insulator layer 115 has a surface 115a having no projections or depressions, so the second insulator layer 115 is substantially flat.
Because the insulator layer 104 in the surface acoustic wave device 101 described in Japanese Unexamined Patent Application Publication No. 2004-112748 has a sufficient thickness over a region where the IDT electrode 103 is present, the insertion loss tends to deteriorate. In addition, when the thickness of the insulator layer 104 is increased in order to improve temperature characteristics, a problem exists in which the fractional bandwidth is reduced.
If, in the acoustic wave device 114 described in WO 2005/034347 A1, the second insulator layer 115 formed on a region where the IDT electrode 113 is present has a reduced thickness to improve the temperature characteristics, it is difficult to improve the temperature characteristics using the second insulator layer 115. On the other hand, if the second insulator layer 115 has a sufficiently increased thickness to improve the temperature characteristics, the thickness of the second insulator layer 115 over the IDT electrode 113 is increased, so a problem in which the fractional bandwidth is reduced exists.
That is, in the surface acoustic wave device 101 or 111 described in Japanese Unexamined Patent Application Publication No. 2004-112748 and WO 2005/034347 A1, there is a problem in which the insertion loss is increased or the fractional bandwidth is reduced when the thickness of the insulator layer over the IDT electrode is sufficiently increased to improve the temperature characteristics. If the thickness of the insulator layer 104 or 115 over the IDT electrode 103 is reduced to avoid this problem, the temperature characteristics cannot be sufficiently improved.